Subfertile Breeding Stallions: Management Strategies (AAEP 2010)

"Stallions do not become sires because of reproductive capability," began Dickson Varner, DVM, MS, Dipl. ACT, professor of large animal medicine and surgery at Texas A&M University. "They're selected based on performance, pedigree, and conformation--reproductive ability is last. The equine breeding industry abounds with stallions whose level of fertility is less than optimal."

Varner discussed several cases of breeding stallion subfertility at the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8 in Baltimore, Md., along with semen and breeding management strategies that effectively increased those stallions' fertility. Some involved live cover programs, while others involved artificial insemination.

"Are we acting unethically when we enhance fertility in stallions?" he asked the audience. "It's not a black and white issue. It is difficult at present, except in isolated circumstances, to differentiate between heritable and nonheritable causes of reduced fertility."

The first step in improving breeding stallion fertility, he said, is to assess a stallion's breeding records to discover the circumstances that result in low fertility. The problem could be with the stallion, the mares, and/or their management. In the case of the mares, for example, he referenced one stallion that had above-average pregnancy rates per cycle for maiden and foaling mares, but lower rates for barren mares (which clearly had lower fertility). That latter group could lead you to underestimate the stallion's fertility if you didn't analyze the mares as well as the stallion.

Management factors can also come into play, and they might depend on the stallion. For example, he described a comparison of two stallions, one with a pregnancy rate that increased with increasing numbers of covers in a day. The other stallion had lower pregnancy rates with increasing number of covers in a day and higher pregnancy rates after periods of sexual rest.

"For the first stallion, the more you breed him the better he gets--you'd want to have a large book of mares and keep some test mares around for him to breed if there are no commercial mares available," Varner explained. "The second would do better with a smaller book."

Other techniques to improve fertility in live cover programs include:

Breeding stallions to test mares in the off season, as many stallions have lower fertility after extended periods of sexual rest.

Using reinforcement breeding (collecting a dismount semen sample, filtering/extending it, and placing this in the mare immediately). Some stallions tend to dismount early, and this can help those stallions considerably (if permitted by the registering organization). This practice also helped one stallion's pregnancy rate following a kick in the groin, until he recovered from that injury. "Overall, it appears that reinforcement breeding can improve pregnancy rates in approximately 60% of Thoroughbred stallions, given the experimental figures available," Varner commented.

Tips for AI Programs

For breeds whose registries allow artificial insemination, there are a few techniques that can improve stallion fertility; Varner focused on centrifugation of sperm and deep-horn low-dose insemination techniques.

When semen is collected for artificial insemination, it is often centrifuged (spun for a short time) to increase the concentration of semen in the end of the tube and thus maximize sperm harvest, as Varner put it. The downside is that sperm can be damaged by this practice, so methods that maximize sperm quality and recovery rates would be ideal. One point he made was that discussions of centrifugation technique should focus on centrifugal forces, not revolutions per minute, as differently sized centrifuges will yield different forces even if spinning at the same RPMs.

Another point was that conical-bottom and nipple-bottom tubes can both be effective for centrifugation, but Varner recommended that nipple-bottom tubes be used when sperm numbers are low or when more seminal plasma needs to be removed.

He also described some cushioned centrifugation strategies that use a cushion medium in the tube, which is "like a little trampoline" that cushions the sperm at the bottom of the tube during centrifugation rather than allowing them to be crushed or suffer concussive injury.

Next, he discussed discontinuous density gradient usage during centrifugation, a process which helps separate sperm that are damaged from those that aren't. The end result, Varner reported, is improved quality of recovered semen; this has been helpful in commercial situations as well as experimental ones.

Finally, he discussed deep-horn, low-dose insemination techniques, which offer the potential to breed more mares per collection by using a dose of semen that can be 200-500 times smaller (in terms of sperm numbers) than a typical dose. This smaller dose is placed at the tip of the uterine horn with a dominant follicle rather than in the uterine body.

"Breeding and semen-manipulation strategies can be applied to maximize the fertility of these stallions and to extend their productive lives," he concluded.

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